The present invention relates to electroless plating methods and to solutions used in the electroless plating of non-conductive or dielectric surfaces.
Chemical plating solutions for depositing metals by autocatalytic chemical reduction of metal ions, in solution and in contact with a catalytic surface of the article to be plated are well known. Such solutions, which do not use electricity, are referred to in the art as electroless plating solutions, and have been found to be of particular use in plating non-metallic substrates such as ceramics and plastics.
Typically, commercial prior art electroless metal plating of non-metallic substrates is accomplished by the steps of treating the substrates either with a sensitizing solution containing stannous chloride followed by treatment with an activating solution containing palladium chloride or other noble metal, or by first treating the substrate with a seeding solution containing both stannous chloride together with a stabilizer followed by treatment with an accelerator solution.
In addition to the attendant costs involved in a process of this nature due to the high cost of palladium, the high catalytic activity of the noble metals is believed to cause contamination of the electroless plating bath, and thus its decomposition. Furthermore, miscellaneous reliability problems have been encountered in printed circuitry technology; some of which are related to the presence of precious metal in the catalytic solution.
In U.S. Ser. No. 521,901, filed Nov. 8, 1974, there is described an improved process for the electroless plating of substrates, especially dielectric or non-metallic substrates which is compatible with present-day commercial procedures and equipment, which does not require the use of palladium or other noble metals, and which specifically includes the step of priming of the substrates with solutions which exhibit strong adhesion and affinity and which may be rinsed without the necessity of a drying step, and yet retain an adsorbed layer which constitutes the basis for the catalytic layer.
In general, the process described in Ser. No. 521,901 comprises the following steps:
A. Priming a dielectric substrate, which has preferably first been cleaned and etched by conventional procedures, by coating the surface of the substrate with an aqueous solution containing stannous and cuprous ions; and
B. Developing the substrate primed by step (A) by reducing the valence state of the cuprous ions present on the surface, preferably by treating the primed substrate with an aqueous solution containing a reducing agent capable of reducing the valence state of the cuprous ions.
While the foregoing process, and the solutions used therein, permit significant cost reductions by the elimination of palladium and other precious metals, processes and solutions permitting even further cost savings would be highly desirable.